Why does the balloon actually rise?

The burner heats the air trapped in the envelope. Hot air is less dense than the cooler air around it, so per the ideal gas law the enclosed gas weighs less than the air it displaces. The buoyant force — the weight of displaced ambient air — then exceeds the balloon's total weight, producing a net upward force.

Why does air density change with altitude?

This simulation uses the barometric formula: ambient air density falls off exponentially with altitude because there is less air column pressing down from above. As the balloon climbs, the ambient air thins out, which reduces the buoyant force even if the burner keeps running at the same setting.

What does the vent control do?

Opening the vent increases the rate at which the envelope's hot air cools toward ambient temperature, lowering buoyancy so the balloon descends — the same mechanic real balloon pilots use to control descent without waiting for the burner to simply stop heating.

Home▸Physics & Mechanics▸Hot Air Balloon Ascent (2D)

Hot Air Balloon Ascent (2D)

A real buoyancy model: burner power heats the envelope's trapped air, the barometric formula thins ambient air with altitude, and the resulting density difference drives a live lift/weight/drag integration you can watch on the readout panel.

Physics & Mechanics2DModerate60 FPS📱 Mobile-adapted⇄ 3D version

This 2D companion replaces the original's decorative sunrise-scene flythrough with an actual buoyancy engine: the envelope's internal gas temperature is integrated against burner heating and passive/vent cooling, ambient air density is computed altitude-by-altitude from the barometric formula, and the resulting buoyant-force-minus-weight-minus-drag balance is what actually moves the balloon — so turning the burner down or opening the vent produces a real, physically consistent change in ascent rate rather than a scripted animation.

⚙ Under the hood

Real buoyancy physics: burner heating and cooling drive envelope gas temperature, the barometric formula sets ambient air density by altitude, and buoyancy, weight and drag are integrated each frame to move the balloon.

buoyancyhot air balloonideal gas lawbarometric formulaconvectiondrag

2D · HTML5 Canvas 2D · 60 FPS target · runs fully client-side, no install